A dihydroxyamino compound is industrially useful and, for example, 1-amino-2,3-propanediol which is one of the dihydroxyamino amino compound is an industrially interesting product as a starting material for a non-ionic X-ray contrast agent (for example, Belgian Pat. No. 855,580), and the demand thereof recently has increased.
For example, JP-A-62230754 (corresponding to U.S. Pat. No. 5,023,379 or DE3609978) discloses a process for the preparation of 1-amino-2,3-propanediol by way of an amine compound from glyceryl aldehyde and ammonia. Further, Swiss patent CH 253256 discloses a process for the preparation of 1-amino-2,3-propanediol by way of an imide compound from acetone-alpha-chlorohydrin and ammonia. Still further, "M. Fedoronko et al., Electroreduction of triose oximes, Chem. Papers 43(2), 335 to 341 (1989)" discloses a process for the preparation of 1-amino-2,3-propanediol by an electroreduction method.
In the processes, it is theoretically thought that 2-amino-1,3-propanediol is not absolutely produced as a by-product, and the processes do not state it at all.
However, until now, the processes have not been industrially put into practice because of high costs in starting raw materials and complicated processes.
Therefore, at present time, 1-amino-2,3-propanediol has typically been industrially produced by a reaction of glycidol or an epihalohydrin with ammonia.
A process for the preparation of 1-amino-2,3-propanediol produced by a reaction of glycidol or an epihalohydrin with ammonia is disclosed in, for example, "Ber. Deutsche Chem. Ges.", Vol. 32, pages 750-757, 1899 (L. Knorr et al) and "Journal of Organic Chemistry", Vol. 27, pages 2231-2233, 1962 (K. Baum et al) and, further, JP-A-56161355 (corresponding to U.S. Pat. No. 4,358,615), JP-A-56161356 (corresponding to U.S. Pat. No. 4,356,323), JP-A-56161357 (corresponding to U.S. Pat. No. 4,360,697), JP-A-04352748 (corresponding to U.S. Pat. No. 5,556,576 or EP 0470004), JP-A-03041056, JP-A-03041057, JP-A-03063251, JP-A-03086851, and JP-A-08012628, etc.
In the processes by the reaction of glycidol or the epihalohydrin with ammonia, it is thought that a reaction mechanism is a ring-opening addition reaction to epoxy group by attack in which electron pair in nitrogen atom of ammonia attacks a carbon atom in epoxy group which is short in electron.
In the processes, 2-amino-1,3-propanediol which is unpreferred for 1-amino-2,3-propanediol is by-produced by a side reaction that ammonia attacks the second carbon from the terminal without an attack to the carbon at the terminal, and impurities such as bis (2,3-dihydroxypropyl) amine are produced by a reaction of 1-amino-2,3-propanediol with glycidol. It is difficult to avoid the side reaction in view of characteristics in the reaction.
It is to be noted that in the processes by the reaction of the epihalohydrin with ammonia, the epihalohydrin produces glycidol in a reaction step of the process.
In the processes described in the above publications, an exceedingly large amount of ammonia is employed in order to increase an yield of 1-amino-2,3-propanediol. A recently commercially supplied 1-amino-2,3-propanediol contains 0.3-0.5% by weight of 2-amino-1,3-propanediol as impurities which are unpreferred in a succeeding step.
However, JP-A-56161355, JP-A-56161356, and JP-A-56161357, JP-A-03063251, JP-A-03041056, JP-A-03041057, and JP-A-03086851 do not specifically state a separation of 2-amino-1,3-propanediol at all.
In the JP-A-04352748 (corresponding to U.S. Pat. No. 5,556,576 or EP 0470004), there is disclosed a process for the preparation of 1-amino-2,3-propanediol containing less than 0.3% by weight of 2-amino-1,3-propanediol by a continuous distillation of a crude liquid of 1-amino-2,3-propanediol with a distillation column equipped with a packing having low pressure loss and a thin-layer evaporator or a scraping-type thin layer evaporator, as a result of finding that 2-amino-1,3-propanediol has, though exceedingly slight, higher boiling point than 1-amino-2,3-propanediol.
However, it is desired that 1-amino-2,3-propanediol is more efficiently separated in the process from a viewpoint of apparatuses, efficiency in using raw materials, and quality of a product.
Further, the JP-A-08012628 proposes that distillation is carried out with a batchwise distillation equipment while streaming an inert gas. However, since 1-amino-2,3-propanediol is thermally unstable, it is not thought that the batchwise distillation is more advantageous than a continuous distillation because of a long holding time of period.
In the circumstances, there has been desired a process in which the content of 2-amino-1,3-propanediol in 1-amino-2,3-propanediol can be efficiently decreased to less than 0.3% by weight.
In the above-mentioned reaction, since the exceedingly large amount of the amino compound is employed in a state of an aqueous solution in order to improve a yield of 1-amino-2,3-propanediol, a large amount of ammonia and water are unavoidably contained in a crude reaction liquid.
Further, discoloring ingredients are readily produced by side reactions in a step for removing ammonia and water from the crude reaction liquid.
JP-A-03086851 discloses that temperature at a bottom portion of an evaporator is controlled in a specified range, and holding time of period is controlled within 1 hour in a reduced distillation step when refined, whereby, there is improved discoloration in a dihydroxyamino compound which is a desired product.
In the processes, refining conditions are limited, and it is difficult to obtain a sufficient yield.
Still further, there is not detailedly described a method for removing discoloring ingredients. Particularly, in the case that the dihydroxyamino compound is employed as a raw material for an X-ray contrast agent, discoloration degree and water content become problematic.
In general, the discoloration degree is roughly controlled in less than 50 of APHA as a target value, and water content is roughly controlled in less than 0.2% by weight as a target value.
However, as described above, there has not been known a method for improving a color hue and water content.
In the situations, there has been desired a development of a method for refining a dihydroxyamino compound without discoloration from a crude reaction liquid.
Also, there are industrially important an improvement in yield of the dihydroxyamino compound and effective recollection and recirculation of unreacted amino compound.
JP-A-03063251 discloses that an ammonia aqueous solution is employed as an amino compound, however, there is not detailedly described recollection of unreacted ammonia. In the processes described in JP-A-56161355, JP-A-56161356, and JP-A-56161357, ammonia is employed as an amino compound, and liquefied ammonia is employed in place of an ammonia aqueous solution in order to reduce the volume of ammonia.
Accordingly, in order to recirculate ammonia into a reaction system, unreacted ammonia must be recollected as liquefied ammonia or as an aqueous solution through a scrubber, etc.
Boiling point of ammonia is -34.degree. C. at ordinary pressure, and in order to recollect ammonia by distillation, temperature of a coolant must be adjusted to less than the boiling point of ammonia, whereby, plant costs and running costs increase, resulting in that it is unpreferred from viewpoint of economy.
Further, there is also thought a process in which distillation is carried out under pressurization in order to elevate the boiling point of ammonia and to elevate the temperature of the coolant.
However, the process requires a distillation column which can resist pressurization, unpreferably resulting in that plant costs increase, and the process requires an apparatus for storing liquefied ammonia.
Still further, in the process for recollecting distilled ammonia as an aqueous solution through a scrubber, etc., recollected ammonia must be converted from an aqueous solution into liquefied ammonia in order to recirculate into a reaction step, unpreferably resulting in that plant costs and running costs become considerably high.
In the circumstances, there has been desired a development of a process in which an amino compound is efficiently recollected from a crude reaction liquid, and recollected amino compound can be recirculated without any special treatments, resulting in that a dihydroxyamino compound can be efficiently prepared.
In addition, JP-A-56161355, JP-A-56161356, JP-A-56161357, JP-A-03041056, and JP-A-03041057 do not state color hue in the dihydroxyamino compound which is a product at all.
In the circumstances, there has been desired a development of a process in which a colorless dihydroxyamino compound is efficiently prepared from a crude liquid after removing low-boiling-point ingredients such as an unreacted amino compound and water from a crude reaction liquid.
The present invention aims at providing a process for efficiently preparing a dihydroxyamino compound in which discoloration is prevented. As a target discoloration value, it is important that an APHA value is less than 50.